Waterjet and abrasive-jet cutting systems are used for cutting a wide variety of materials. In a typical waterjet cutting system, a high-pressure fluid (e.g., water) flows through a cutting head having a cutting nozzle that directs a cutting jet onto a workpiece. The cutting nozzle may include a mixing tube for introducing an abrasive into the high-pressure cutting jet to form an abrasive cutting jet. The cutting nozzle may then be controllably moved across the workpiece to cut the workpiece into the desired shape. After the cutting jet (or abrasive cutting jet) passes through the workpiece, the energy of the cutting jet is dissipated and the fluid is collected in a catcher tank for disposal. Waterjet and abrasive jet cutting systems of this type are shown and described, for example, in U.S. Pat. No. 5,643,058 issued to Erichsen et al. and assigned to Flow International Corp. of Kent, Wash., which patent is incorporated herein by reference. The '058 patent corresponds to Flow International's Paser 3 abrasive cutting systems.
FIG. 1 is an isometric view of a waterjet cutting system 10 in accordance with the prior art. The waterjet cutting system 10 includes a cutting head 20 coupled to a mount assembly 30. The mount assembly 30 is controllably driven by a control gantry 40 having a drive assembly 42 that controllably positions the cutting head 20 throughout an x-y plane that is substantially parallel to a surface 14 of a workpiece 12. Typically, the drive assembly 42 may include a pair of ball-screw drives oriented along the x and y axes and a pair of electric drive motors. Alternately, the drive assembly 42 may include a five axis motion system. Two-axis and five-axis control gantries are commercially-available as the Bengal 4×4 cutting systems from Flow International of Kent, Wash.
FIG. 2 is a partial-elevational side view of the cutting head 20 and the mount assembly 30 of the waterjet cutting system 100 of FIG. 1. The cutting head 20 includes a high-pressure fluid inlet 22 coupled to a high-pressure fluid source 50, such as a high-pressure or ultra-high pressure pump, by a high-pressure line 23. In this embodiment, the cutting head 20 includes a nozzle body 24 and a mixing tube 26 terminating in a jet exit port 28. Although the term “mixing tube” is commonly used to refer to that portion of the cutting head of an abrasive jet cutting system in which abrasive is mixed with a high-pressure fluid jet to form an abrasive cutting jet, in the following discussion, “mixing tube” is used to refer to that portion of the cutting head 20 that is closest to the workpiece 12, regardless of whether the waterjet cutting system uses an abrasive or non-abrasive cutting jet.
The mount assembly 30 includes a mounting arm 32 having a mounting aperture 34 disposed therethrough. The mounting arm 32 is coupled to a lower portion 44 of the control gantry 40. The nozzle body 24 of the cutting head 20 is secured within the mounting aperture 34 of the mounting arm 32.
In operation, high-pressure fluid from the high-pressure fluid source 50 enters the high-pressure fluid inlet 22, travels through the nozzle body 24 and mixing tube 26, and exits from the jet exit port 28 toward the workpiece 12 as a cutting jet 16. The cutting jet 16 pierces the workpiece 12 and performs the desired cutting. Using the control gantry 40, the cutting head 20 is traversed across the workpiece 12 in the desired direction or pattern.
To maximize the efficiency and quality of the cut, a standoff distance d (FIG. 2) between the jet exit port 28 of the mixing tube 26 and the surface 14 of the workpiece 12 must be carefully controlled. If the standoff distance d is to close, the mixing tube 26 can plug during piercing, causing system shutdown and possibly a damaged workpiece 12. If the distance is too far, the quality and accuracy of the cut suffers.
The mixing tube at 26 is typically fabricated of specially formulated wear-resistant carbides to reduce wear. Particularly for abrasive cutting systems, the mixing tube 26 suffers extreme wear due to its constant contact with high velocity abrasives. Thus, mixing tubes are a relatively expensive component of the cutting head 20. The specially formulated carbides are also quite brittle, and can easily break if the mixing tube 26 collides with an obstruction during operation of the cutting system 10, such as fixturing or cut-out portions of the workpiece 12 which have been kicked up during the cutting operation. Accidental breakage of the mixing tube 26 increases operational costs and downtime of the cutting system 10.
Current collision sensors use a ring sensor disposed about the mixing tube 26 which slides along or slightly above the surface 14 of the workpiece 12. The ring sensor indicates the relative height of the workpiece. A motorized ball-screw drives the cutting head up and down to maintain the required standoff distance. When the ring collides with a kicked-up part or other obstruction, a detector detects the collision and sends a stop signal to the control gantry to stop the movement of the mixing tube in an attempt to avoid the collision.
A fundamental problem with such collision sensors is that they must have a large enough “safety buffer” between the sensor and a mixing tube to allow the control gantry enough time to stop without damaging the mixing tube. Due to the size and speed of modem cutting systems, the task of stopping the control gantry quickly to avoid a collision is quite difficult. Another problem is that any shifting of the components requires a lengthy re-calibration routine to insure proper standoff distance d. A serious collision can ruin the ring sensor.
One approach has been to simply make the ring larger the allow to control gantry more room to stop. This approach, however, prevents the cutting jet 16 from cutting near obstructions and fixtures commonly found around the edges of the workpiece 12, thereby wasting material. Enlarging the ring also increases the occurrence of erroneous collision signals which results in unnecessary downtime of the cutting system. Finally, existing ring sensor devices are expensive and are not robust in detecting surface height or collisions when operating the control gantry at high-speed or under dirty conditions.